LBL scientists are using scanning tunneling
and atomic force microscopes to image the arrangement of atoms on
surfaces. In the future, these instruments may be used to manipulate
individual atoms.

Miquel Salmeron learned about the development of the scanning tunneling
microscope shortly after it was invented a decade ago. Gerd Binnig and
Heinrich Rohrer of the IBM Research Laboratory in Switzerland had
developed a new device capable of imaging and even manipulating details as
small as a single atom.

"Right away, those of us who are surface scientists knew this was
a revolutionary breakthrough," recalls Salmeron, a researcher in
LBL's Materials Sciences Division. "The characteristics of a
material--its reactivity, its mechanical properties, its behavior in an
electronic device--largely are determined by the atomic structure of its
surface.

"Researchers had been using all kinds of techniques for 20 years
to determine the atomic structure of surfaces. None allowed us to see
individual atoms, with the sole exception of the field ion microscope for
imaging a few metals. Suddenly, the scanning tunneling microscope arrived.
For the first time, we could see atoms on any conductive surface."

By 1986, five years after its invention, the scanning tunneling
microscope (STM) had become such a vital tool in materials science and
chemistry that Binnig and Rohrer were awarded the Nobel Prize in Physics.
The instrument creates images using a radically different means than
conventional light or electron microscopes. It uses a sharp tip, almost
like that of a phonograph stylus, and as it scans over an object, it maps
out a three-dimensional image.

Salmeron began working with the STM in 1985. Today, he heads an LBL
Center for Advanced Materials research program that has as its hub the STM
and a related offspring instrument, the Atomic Force Microscope (AFM).
Salmeron and collaborators Frank Ogletree, Joe Katz, and William Kolbe
have developed unique versions of STM and AFM microscopes, along with
advanced electronic controls and computer software. Several of these
advances have found their way into commercial microscopes.

At LBL, the STM and AFM are vital components of more than a dozen
current research projects. The work includes the study of the surface
structure of catalysts under reaction conditions, explorations of the
feasibility of mapping and sequencing DNA, studies of the atomic-scale
mechanical properties of lubricated surfaces, examinations of
semiconductor interfaces, and the use of lasers to capture and image
extremely fast, nanometer-scale interactions.

A decade ago, the term nanometer (a billionth of a meter) was not
common parlance in the high-technology community. By making
nanometer-scale objects accessible, the STM and AFM are playing a major
role in the ascendance of nanometer-scale science and technology.

To understand how the two instruments actually work, one needs to
discard the usual notion of a microscope. The STM and AFM do not create
images the way a conventional microscope does. They rely on what might
loosely be called touch. Indeed, one STM scientist, Carlos Bustamante of
the University of Oregon, calls the instruments "a kind of Braille
way of looking at molecules."

The STM creates an image by scanning a tip across a sample. A computer
records the precise location of the tip and generates an image based on
the path of the tip as it moves above a sample at a constant height.

The device owes its existence to a quantum mechanics effect in which an
electron can penetrate an energy barrier. According to quantum mechanics,
explains Salmeron, the wave function of an electron extends out beyond the
surface of any material. The STM takes advantage of the overlap between
the electron wave functions surrounding the object to be imaged and the
wave functions of the electrons associated with the STM's tip. When the
tip is moved to within a few angstroms of a sample's surface, a current of
electrons crosses the gap--a phenomenon known as "tunneling."
(The sample must be conductive or affixed to a conducting substrate.)

Because tunneling is very sensitive to the distance between the tip and
the sample, the STM can map out the surface of the sample with great
precision. In one mode, researchers use an electrical feedback mechanism
to maintain the tip at a constant distance as it scans over the sample, up
and down over surface contours as slight as single atoms. Topographical
data are recorded and translated into a three-dimensional image by a
computer.

The AFM, a descendant of the STM, does not require that the sample or
substrate be conductive. The instruments are similar in that they both use
a tip to sense the atomic shape of a sample as well as a computer to
record the path of the tip and generate an image. However, the AFM tip
"touches" the sample. In a phonograph needle, about one gram of
force is applied on the stylus; in the AFM, the load on the tip is about
one ten-millionth of a gram, a force so slight that it does not dislodge
even a single atom.

Translated into quantum mechanical terms, the electron wave functions
of the tip and the sample being imaged can overlap strongly enough to give
rise to strong repulsive forces. Thus, in the AFM, the tunneling current
upon which the STM relies is replaced by the force between the tip and the
sample. The instrument can be adapted to sense a range of forces,
including attractive or repulsive, interatomic, electrostatic, and
magnetic forces. These forces cause angstrom-size displacements of the
cantilevered tip, movement which is recorded and translated into a
topographic image.

Though nanometer-scale engineering is a potential use of the STM and
AFM, atomic-resolution imaging remains the current focus of the LBL
program. "We are interested in studying surface problems like the
atomic-scale mechanical properties of surfaces," says Salmeron.
"Clearly, the information derived from these microscopy studies is
providing new insights."

Surfaces are critical in the performance and interaction of materials.
To improve performance and to create new materials, scientists study
surfaces, attempting to learn what happens at an atomic level. Until the
advent of the STM and AFM, surface science took place in a vacuum.
Literally.

Explains Salmeron, "Surface science has relied on analytical
techniques that only work under high vacuum conditions. Over the years,
scientists have done these studies under vacuum and hoped they were still
relevant to the real world. This has been a major impediment.

"However, the STM and AFM don't require a vacuum. We are now
imaging surface reactions under normal atmospheric pressures as well as at
high pressures. The pressures can range from one to 100
atmospheres--whatever pressure and temperature are typically used for the
reaction we are studying. We are the first in the world to do this."

In collaboration with Gabor Somorjai, who heads LBL's Surface Science
and Catalysis Program, and UC Berkeley graduate student Brian McIntyre,
the researchers are using the new high-pressure STM techniques to study
platinum, the most versatile and common catalyst. Platinum, for instance,
is used in catalytic converters in automobiles to reduce exhaust
emissions.

Nobody has ever seen the atomic arrangement of a platinum surface as it
catalyzes reactions in the exhaust gas. Scientists know that surface
reactions can be almost cataclysmic, like an earthquake warping the entire
surface. And they know that platinum makes the reaction go. But they don't
know exactly why or how it works. Observing each step in the reaction
might very well allow scientists to alter and improve the reaction.

AN STM IMAGE REVEALS THE TOPOGRAPHY OF A GRAPHITIC LAYER OF
CARBON ON PLATINUM, PRODUCED BY HEATING THE PLATINUM CRYSTAL IN
ULTRAHIGH VACUUM TO MORE THAN 1300 KELVIN

So far, LBL researchers have examined the platinum surface under
reaction pressures before and after the adsorption of carbon monoxide,
hydrogen, oxygen, and sulfur. Under atomic-resolution magnification, they
have been able to observe changes in the atomic structure due to the
reaction conditions.

"Chemical reactions often happen on the irregularities, the
defects of a surface," explains Salmeron. "Chemical activity is
greatest where atoms of platinum are missing or at steps in the material.
We are looking for these magical places where the reaction proceeds.

"Already, we have observed the migration and diffusion of sulfur
on the surface. Ultimately, we want to see the reaction as it proceeds at
the atomic scale. We want to look at a long list of catalysts and trace
the fate of individual atoms throughout the course of the reaction."

The epitaxial or layered growth of materials is another field of
chemistry that is under investigation with the new microscopes. LBL
researchers are examining the growth of metal oxides on catalysts--for
example, the growth of titanium oxide and iron oxide on platinum--and
studying how islands of these metal oxides modify the catalytic properties
of platinum. Because titanium oxide and iron oxide are not by themselves
catalysts, this effect is not understood.

Through microscopic images, the researchers have viewed islands made of
a few atoms of these oxides and noted that titanium atoms at the
perimeters of these patches have higher tunneling probability, apparently
because they are less oxidized than atoms at the center. This observation
suggests that the edges of these metal oxide islands may be responsible
for the enhancement of certain catalytic reactions.

In the field of electronics, Salmeron's group is investigating how to
improve gallium arsenide semiconductors in a collaboration with Eicke
Weber, an LBL researcher and professor in UC Berkeley's Materials Science
and Mineral Engineering Department, and graduate student Jun Fei Zheng. To
accomplish this, they are using an STM with a tip that can be navigated.
Rather than beginning its mapping of the atomic landscape at a random
point, the "Johnny Walker" STM (so named because it can walk
around) can be steered to the interfaces between different semiconductors
or metals.

"These interfaces are critical in microprocessors," says
Salmeron. "You have to have a clean flow of electrons. Different
semiconductor materials have different electronic properties, which depend
upon their atomic structure. We want to know if there are more defects,
more missing or excess atoms, as you approach the interface.

"Now, we can see exactly what defects have been introduced by
different aspects of the fabrication process. Chemical treatments, the
type of metal contacts, the effect of the heat used to create interfaces
on the nearby current carriers in the semiconductor -- all these processes
can alter electron flows. We hope to be able to identify which processes
create adverse changes and which create the most favorable
interfaces."

Just as with a light microscope, researchers can vary the level of
magnification of an STM or AFM image. The tips of both instruments scan
across a sample, propelled by a piezoelectric driver which mechanically
moves the tip when it is stimulated by an electric signal. The more
voltage, the greater the movement of the tip, and the larger the area
being imaged. One-tenth of a volt, for instance, moves a tip about one
angstrom or the width of a single atom.

Ogletree says that for scientists seeking to observe what is happening
at an atomic scale, the STM and AFM provide "instant
gratification." Snapshots of a surface can be created in one to two
seconds.

Imaging with the STM and AFM is similar to conventional photography in
terms of the effects of long exposures. The faster the image is created,
the less likely the subject will move and cause the image to be blurred.
In the case of the microscopes, surface temperature differences affect the
motion of the tip. The longer the tip takes to scan a sample, the greater
the distortion.

Several seconds can elapse during the creation of an image without
thermal distortion occurring. Through a succession of images, scientists
can even observe atomic-scale events evolving on a surface. However, the
microscopes have not been nearly fast enough to record intermediate
reactions, events that occur on a lightning-like, picosecond (trillionth
of a second) timeframe.

Through the use of lasers, the scientists hope to be able to witness
intermediate reactions.

Currently, pairs of lasers are in common use as tools to measure the
dynamics of surface reactions. After a pump laser stimulates a surface, a
probe laser is used to measure the event. In collaboration with LBL
Materials Sciences Division Director Daniel Chemla's group, the
researchers are working on a process called time-resolved imaging, in
which lasers are paired with the STM and AFM.

"The general concept is to use one laser to modify a surface and a
second, short-pulsed laser as a switch that turns the STM on and off in
picoseconds," explains Ogletree. "While turned on, the STM is
poised over a single point and stays there for a number of cycles to make
sure its measurement is accurate. This process would be repeated over
every point in a grid that is under examination. The result should be an
atomic-resolution image of an intermediate stage that has a lifespan
measured in picoseconds."

Ogletree says one planned experiment would start with an undoped
semiconductor--one without added impurities. Thus, the semiconductor would
be without current carriers. Pulses of laser light would be used to create
very short-lived carriers.

"The precise progress of a pulse of electrons across a
semiconductor is really an unknown," says Ogletree. "The
velocity of these current carriers is about one angstrom per femtosecond
(a millionth of a billionth of a second). With time-resolved imaging, we
believe we can observe how an electron makes its way across a
semiconductor. We can see how the pulse of electrons spreads as it moves
and just how."

Recently, the AFM has begun to make advances into another realm of
science, that of biology.

Back in 1987, Salmeron's group used the STM to create the first images
ever produced of native DNA. These images showing the double helix
suggested that the STM might be used in the Human Genome Project, which
aims to map and sequence human DNA. Reproducing those first images of DNA,
however, has proved to be a challenge.

The STM requires either a conductive sample or a sample anchored to a
conductive substrate. DNA is not conductive, and binding it to the
substrate again has proved to be difficult. When the tip approaches a
piece of DNA, the strong current exerted seems to dislodge the sample and
push it aside. Salmeron says one way to keep the sample from slipping away
is to raise the STM tip further above the sample. The downside to this
approach is that it lowers the current level and lengthens imaging time to
a blurry six minutes.

Researchers have turned to the AFM, an instrument ideally suited to
imaging nonconductive samples, and have now mastered the technique of
imaging DNA with it. UC Berkeley biophysics graduate student Matthew
Murray can image DNA routinely but, so far, not at a resolution where
individual nucleotides can be discriminated.

The LBL team has become the first to demonstrate that DNA labeled with
proteins can be imaged with the AFM. Segments composed of 353 and 701 base
pairs of nucleotides have been marked with a protein and imaged. Murray
says this demonstrates that the AFM can be used to measure the distance
between two points of DNA and to map DNA--that is, determine the
chromosomal location of specific genes.

Murray points out two important advantages to using the AFM for
mapping. Compared to alternative techniques, such as gel electrophoresis,
very little DNA is required, and the AFM is faster, producing an image in
one minute as opposed to one hour.

Could the AFM ever be used to reveal the sequence of the three billion
nucleotides that compose the human genome?

Several obstacles stand in the way. Segments of DNA that consist of
less than 1000 pairs of nucleotides are relatively straight. But longer
strands bind up into a tangled pile, much like snarled fishing line.
Tangled DNA cannot be measured nor can the interior realm of the tangle be
imaged. On the other hand, the distance between one nucleotide and another
is about three angstroms, and the AFM can image details as small as one
angstrom. Murray says that if a method can be devised to prevent longer
strands of DNA from tangling, biologists will want to reassess the
feasibility of using the AFM for sequencing.

Rohrer, the co-inventor of the STM, has suggested that the
"M" in STM and AFM really should be changed. Instead of
microscope, he says, the "M" ought to stand for method. The
instruments actually represent new methods for working on the nanometer
scale. "By work," he wrote, "I mean observing a single,
individually selected nano-object, measuring and understanding its
properties, manipulating it, modifying it, and ultimately observing and
controlling its possible functions and related processes."

Salmeron recalls a widely publicized experiment that demonstrates some
of the possibilities suggested by Rohrer. One year ago, IBM scientists
used individual atoms of xenon to spell out the IBM logo. While clever,
this was no idle stunt.

Says Salmeron: "Many scientists are researching how to use the
devices to make holes or mounds in substrates, creating a surface that can
store digital information. Each hole or mound is the size of several
atoms.

"Think about the amount of information you could store with these
devices. Say each letter requires a space the size of 10 atoms, or 100
angstroms. You could put one trillion letters in a square centimeter. That
amounts to over one million books, each with hundreds of pages."

Miniaturization has been a constant force in the march of technology,
and Salmeron is convinced that the bounds of progress will be further
broadened by the STM and AFM.

"There is no technological impediment to mechanizing the STM and
AFM, to using the instruments as computer-driven robots for atomic
manipulation. The tips can be used as atomic scalpels. You can literally
move around individual atoms. Like many scientists," says Salmeron,
"I believe that one day we will do atomic engineering with these
devices."